Recent advances in nanotechnology create a demand for greater scientific understanding of the transient ballistic phonon transport at the nanoscale. It is believed that ballistic phonons may travel for long distances without destruction, but it is unclear how far they can travel. Here, a numerical model is developed to study phonon transport in silicon nanofilms. It is elucidated how thermal pulses are transmitted in silicon nanofilms by longitudinal, ballistic transverse and dispersive transverse phonons. It is found that both ballistic longitudinal and ballistic transverse phonons are highly dissipative so they can only travel for short distances, while dispersive transverse phonons at lower frequencies are less dissipative and can travel for longer distances. There exists a similarity parameter (Knudsen number) in thin-film heat conduction with different thicknesses.

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